1. Field of the Invention
This invention generally relates to nuclear meters, and particularly to an apparatus or a method for analyzing the spectral data and for reducing the deleterious effects caused by gamma rays from interfering elements or other effects in a neutron-capture-based elemental analyzer for on-line measurement of bulk substances.
2. Description of the Prior Art
The rising cost of fuels, coupled with the need to avoid atmospheric pollution when burning them, has led to the requirement that their composition be known at various points in the fuel-preparation cycle. For example, because of the scarcity of low-sulfur crude oils and the cost of sulfur removal, the value of fuel oil increases significantly as its sulfur content becomes lower, indicating that accurate fuel-oil blending to a fixed sulfur level consistent with allowable amounts of pollution is both cost effective and efficient utilization of increasingly-scarce hydrocarbons. Furthermore, precise knowledge of the heat content of fuel oil allows furnaces and boilers to be operated in a more efficient manner. In addition, knowledge of the amount of sulfur and other contaminants such as vanadium and nickel in various hydrocarbon streams can help prevent the poisoning of catalysts used in oil refineries, avoiding costly shut downs.
In the case of coal, sulfur content is generally higher than that of oil, making the pollution problem even more severe. As a result, expensive coal-cleaning plants, stack-gas scrubbers and precipitators are necessary, all of which can be operated more efficiently if the coal composition is known on a real-time on-line basis. Efficient boiler operation also benefits from this composition measurement, and knowing the composition of the ash in the coal can be used to avoid boiler slagging.
Because these composition measurements have to be made on inhomogeneous substances with high mass flow rates and variable compositions, the measurement should continuously reflect the average composition of the bulk substance. Response times should be fast enough to permit effective process control, which generally implies a speed of response ranging from a few minutes up to an hour.
A technique which can satisfy these requirements can often be used in applications which do not involve fuels or their derivatives. For example, it could measure the nitrogen content of wheat in order to determine the amount of protein present, which in turn is related to food value. Thus, the measurement of fuels is illustrative only and is not essential to this invention, which applies to all measurements of bulk substances by the techniques to be described hereinafter.
Several methods for composition measurement are known in the prior art, the most obvious one being sampling followed by chemical analysis. This technique provides most present data on the composition of various bulk substances. Unfortunately sampling is inherently inaccurate because of the lack of homogeneity of bulk materials, and large continual expenditures for manpower, sampling devices and chemical-analysis equipment are required to provide response times which at best could approach one hour. These disadvantages lead to the consideration of other techniques which are faster, more subject to automatic operations and more of an on-line continuous bulk measurement.
One technique often used in industrial environments for elemental analysis involves X-ray fluorescence. This technique relies on the fact that each atom emits X rays with distinct and well-known energies when external radiations disturb its orbital electrons. Unfortunately, sulfur, which is an interesting element from the standpoints of air pollution and catalyst poisoning, emits mostly 2-keV X rays, which can only traverse about 0.1 mm of a typical fuel. Iron, which is one of the elements generating the highest-energy X rays in coal, produces mostly a 6-keV X ray, which also cannot escape from any appreciable amount of coal or other nongaseous fuel. Thus, the use of X-ray fluorescence for other than gaseous materials requires either the preparation or the vaporization of a sample in an atmosphere which does not confuse the measurement. In either case, a difficult sampling and sample-preparation problem compounds the errors associated with X-ray fluorescence itself.
A second technique usually involving X rays which are more penetrating is X-ray absorption. In this case one measures the differences in the absorption or scattering of X-rays caused by changes in the amounts of certain elements. In the case of relatively-pure hydrocarbons such as refined fuel oil, this technique can provide a useful measurement of sulfur content because sulfur at X-ray energies near 22 keV can have a predominant effect on the X-ray absorption. This predominance, however, is dependent on the lack of most of the metals which are present in coal and may also be present in oil. In addition, 22-keV X rays only penetrate about 2 mm in most non-gaseous fuels, making sampling still a requirement. Moreover, this technique is generally limited to measuring only one of several potentially interesting elements, and the measurement of the relative amounts of many different elements in a complex mixture such as coal becomes difficult.
Nonetheless, nuclear techniques in general remain attractive because they often can be automated and in principal do not require actual manipulation of the bulk material itself. The problems with X-ray fluorescence and absorption arise partly because the associated radiations are not sufficiently penetrating. However, because the energetic gamma rays produced by the capture of thermal neutrons will penetrate over 100 mm of most fuels, an analysis technique based on them can provide an accurate, continuous, on-line measurement of the elemental composition of bulk substances without sampling.
This technique is based on the fact that almost all elements when bombarded by slow neutrons capture these neutrons at least momentarily and form a compound nucleus in an excited state. Usually the prompt emission of one or more gamma rays with energies and intensities which are uniquely characteristic of the capturing nucleus dissipates most of this excitation energy. Because these prompt gamma rays often have energies in the 2- to 11-MeV range, they can penetrate substantial quantities of material to reach a gamma-ray detector and its associated electronics which provide a measurement of their energy spectrum. Thus, for those isotopes with significant capture cross sections and prominent gamma-ray lines, measurement of the number of prompt gamma rays present at various energies can be used to determine in an on-line, real-time basis the quantity of most of the elements present in bulk substances, which can be flowing through the analyzer.
Although this technique has been used in the laboratory under controlled conditions, its implementation in an automatic, on-line instrument placed in an industrial environment presents unique problems. One of these problems result from the need to measure accurately the concentration of one element in the presence of varying concentrations of other elements. Because most elements produce capture gamma rays, the measured energy spectrum can become complex when several elements are present in the bulk substance being analyzed or in structural materials exposed to the neutrons. Moreover an interesting element may be present in both the substance being analyzed and in the structural materials, leading to the problem of distinguishing between detected gamma rays from these two sources.
In addition most elements produce many gamma rays with differing energies, and each one of the energetic gamma rays produces in turn three peaks in the pulse-height spectrum because of the escape of none, one or two positronannihilation photons. Because of the finite energy resolution of the detector, these peaks may interfere with each other, so that in the composite spectrum of a low-resolution detector such as NaI(Tl) few peaks arise purely from a single gamma ray, and simply counting events near a line from an interesting element seldom produces an accurate measurement of the concentration of that element.
This problem becomes more complex because these peaks ride on a slowly-varying continuum mostly generated by Compton scattering in materials both within and outside of the detector. Although the contribution of the detector to the continuum yields a fixed ratio of a peak height to the Compton continuum, the contribution of the medium outside of the detector depends on its density and neutron-moderation properties. Additionally pulse pileup, gamma rays from the source and shielding materials, and neutron reactions in the detector and its housing can add to the continuum and can possibly also produce undesirable peaks in the spectrum.
Even if the peaks are separated from the continuum and after various interferences are resolved, absolute counting rates are still not useful directly. For example, an isotopic neutron source will decay. Additionally the predominance of hydrogen and trace elements with large neutron-capture cross sections in the capture of neutrons makes the absolute rates overly sensitive to the amount of these elements present. Moreover, the size of the neutron cloud, determined by the moderating properties and density of the region around the source and detector, effects absolute rates and the peak-to-Compton ratios.
Prior-art instruments used for measuring the sulfur content of coal attempted to overcome these problems by dividing the energy spectrum into two regions. One of these regions included the peak near 5.43 MeV produced by gamma rays from sulfur, and the other region was immediately adjacent to the first region. By subtracting the number of counts in the second region from the number of counts in the first region, the resulting difference could be made relatively independent of the amount of iron present in the coal.
However, this technique was effective only over a small range of elemental concentrations and provided no immunity for changes in other elements such as nitrogen. In addition only the concentration of sulfur could be measured, and errors in this measurement arose when the carbon-hyrogen ratio in the coal changed. Thus, the prior-art technique for analyzing the spectral data was too simplistic for a general-purpose elemental analyzer, which must perform accurately even if the composition of the bulk substance changes.